1. Field of the Invention
The present invention relates to an apparatus for controlling an internal combustion engine, which is also referred to hereafter simply as an engine. More particularly, the present invention relates to an apparatus for controlling an engine having a variable-valve mechanism.
2. Related Art
In the conventional engine, a throttle valve is provided on an intake pipe of the engine. The throttle valve adjusts the opening thereof in order to control an intake airflow. The driver depresses an accelerator pedal connected to the throttle valve by an electrical link mechanism so that the valve operates in accordance with a pedal-depression quantity. In addition, the throttle valve can also be controlled by an electrical control unit and a motor. The control of intake airflow executed by using the throttle valve is referred to as the throttle-valve control. Since a volume exists between the throttle valve and a cylinder, a response to a control command in the control of the intake airflow lags behind the command. In addition, a negative pressure is built up the downstream of the throttle valve. For this reason, a relatively large pumping loss is incurred.
The engine also has an intake valve and an exhaust valve. The intake valve and the exhaust valve are driven by a valve-driving mechanism such as a cam or by an electrical actuator. Operating characteristics of al least one of the intake valve and the exhaust valve are prescribed in terms of its attributes such as an opening timing, a closing timing, a valve opening, a valve lift quantity and a lift-quantity waveform. There is known a variable-valve mechanism for varying the operation characteristics of the valves. For example, there is a variable-valve mechanism for adjusting the opening and the closing timings in an advance or retard direction. Another example is a variable-valve mechanism for adjusting the opening of the valve to a value between a zero and a maximum. Another typical variable-valve mechanism adjusts the operation characteristics with a high degree of freedom. In an engine having such a variable-valve mechanism, the intake airflow can be adjusted by using the variable-valve mechanism. The control of intake airflow executed by the variable-valve mechanism is referred to as variable-valve control. Typically, the operation characteristics of the valve are adjusted in accordance with an acceleration operation quantity and the operating state of the engine. The variable-valve control generates a small response lag in comparison with the throttle-valve control. In addition, in the case of the variable-valve control, the magnitude of an incurred pumping loss can be reduced. For example, by execution of variable-valve control, the throttle valve can be opened relatively. In a typical engine, the execution of variable-valve control makes it unnecessary to install a throttle valve.
JP-A-8-193531 discloses an apparatus for automatically stopping the engine temporarily. Such an apparatus is referred to as an automatic stop and start apparatus or an idling stop control apparatus. Control executed by the apparatus is known as automatic stop and start control. When the vehicle is halted, for example, the engine is automatically stopped without the need for an operation to be carried out by the driver. Such control is referred to as automatic stop control. When the driver makes an attempt to drive the vehicle after the automatic stop control, the engine is automatically started. In response to an operation carried out by the driver to depress the accelerator pedal, for example, a start motor is automatically activated to start the engine automatically. The start motor can also be automatically activated to start the engine when the driver carries out an operation to release the brake pedal. Such control is referred to as automatic start control or automatic restart control. The automatic stop and start control is a means capable for effectively reducing the fuel consumption, exhaust emissions and noises.
By execution of the automatic stop and start control, on the other hand, a transient state such as the start or stop of the engine occurs very frequently. For this reason, there is demanded proper control of the engine also in the transient state such as the start or stop of the engine.
Assume for example that, in automatic stop control, a valve operation characteristic prior to the automatic stop control is saved. In this case, in the next automatic start control based on the saved valve operation characteristic, it is feared that a smooth start of the engine is obstructed.
As another example, assume that the lift quantity of the exhaust valve is set at a large value in automatic stop control. In this case, residual gas left in the engine flows out from the cylinder to the exhaust pipe when the engine is halted temporarily. In particular, in an inactivated state of a catalyst for cleaning exhaust gas, the state of the exhaust emissions is worsened.
For example, the intake change of intake airflow resulting from the variable-valve control can be detected by an intake-air-flow sensor or an intake-airflow meter only after a fixed delay. Right after the engine has been automatically started, on the other hand, the operating state of the engine changes abruptly. Thus, there is a case in which, by execution of the variable-valve control, the intake airflow cannot be adjusted properly. As a result, a torque shock is generated. In addition, a change in air-fuel ratio is resulted in.
In a process wherein the engine speed becomes lower than an idle speed due to the automatic stop control, for example, the engine speed temporarily matches the characteristic frequency of the engine itself or the characteristic frequency of the driving system of the vehicle. As a result, resonance occurs, temporarily increasing the amplitude of vibration and the magnitude of noise.
In the automatic stop control, for example, a starter is used frequently. As a result, there appears a tendency to reduction of the battery voltage. In particular, the voltage of the battery decreases at an automatic-start time. When the voltage of the battery decreases, the variable-valve mechanism does not operate in a stable manner in some cases. When the voltage of the battery decreases, for example, it is quite within the bounds of possibility that the operation characteristic of the valve cannot be controlled to follow a target operation characteristic. As a result, it is feared that the exhaust emissions deteriorate.